Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. The concepts discussed in this section are known to the inventors but are not necessarily known to others or previously conceived or pursued by others.
The three primary functional components of a lithium-ion battery are a negative electrode, a positive electrode, and an electrolyte. The negative electrode, or anode, of a conventional lithium-ion cell comprises a material that is capable of absorbing or releasing lithium ions, and is typically made from carbon, such as graphite. The positive electrode, or cathode, comprises a lithium-containing material and is typically made from one of the following types of lithium-containing materials: a layered oxide (such as lithium cobalt oxide), a polyanion (such as lithium cobalt phosphate), or a spinel (such as lithium manganese oxide). The electrolyte, which is in communication with the electrodes, is a lithium salt in an organic solvent. The electrochemical roles of the electrodes change between anode and cathode, depending on the direction of current flow through the cell. Typically, the battery also includes a separator, which is a microporous membrane, to prevent contact between the anode and cathode.
Much attention has been focused on the chemistry of the cathode active materials since the selection of the cathode active material has a major impact on the voltage, capacity, life, and safety of a lithium-ion battery. Presently, the layered-layered lithium-rich cathode material xLiMO2.(1-x)Li2MnO3, where M is a transition metal, such as Ni, Mn, Co, has shown great promise for use in lithium-ion batteries for electric vehicles. In fact, layered-layered lithium rich cathode material possesses high specific capacity (250 mAh/g) which is 70% higher than the commercially-used lithium nickel manganese cobalt oxide LiNi1/3Mn1/3Co1/3O2. However, due to the intrinsic oxygen release upon formation, and reactivity with the electrolyte solvents, the cycling performance and rate capability of the layered-layered lithium rich cathode material are compromised. As a consequence, both surface and bulk material have high impedance.
There is, therefore, a need, for a means to improve the performance of lithium-ion cathode material, and in particular layered-layered lithium-rich cathode material.
Since the primary solvents in the electrolyte are cyclic and linear carbonates, their oxidative reactions with the cathode surface lead to irreversible losses and severe capacity fading. There have been attempts in the prior art to control the electrode/electrolyte reactivity. One approach, as reported by Achiha, et al., J. Electrochem. Soc., Vol. 156, pages A483-A488 (2009), has been to replace the highly reactive carbonate-containing solvents with more stable ones that can be fluorinated. A second approach, as reported in Han, et al., J. Power Sources, Vol. 187, pages 581-585 (2009), has been to use additives in the electrolyte that can form a protective layer on the oxidized electrode surface. Yet another approach has been to coat the cathode surface with a material that will inhibit the reactivity of the oxidized electrolyte with the electrode surface, while allowing lithium-ion conduction.